Isomeric Effects of Au28(S-c-C6H11)20 Nanoclusters on Photoluminescence: Roles of Electron-Vibration Coupling and Higher Triplet State

The exploration of near-infrared photoluminescence (PL) from atomically precise nanoclusters is currently a prominent area of interest owing to its importance in both fundamental research and diverse applications. In this work, we investigate the near-infrared (NIR) photoluminescence mechanisms of two structural isomers of atomically precise gold nanoclusters of 28 atoms protected by cyclohexanethiolate (CHT) ligands, i.e., Au28i(CHT)20 and Au28ii(CHT)20. Based on their structures, analysis of 3O2 (triplet oxygen) quenching of the nanocluster triplet states, temperature-dependent photophysical studies, and theoretical calculations, we have elucidated the intricate processes governing the photoluminescence of these isomeric nanoclusters. For Au28i(CHT)20, its emission characteristics are identified as phosphorescence plus thermally activated delayed fluorescence (TADF) with a PL quantum yield (PLQY) of 0.3% in dichloromethane under ambient conditions. In contrast, the Au28ii(CHT)20 isomer exhibits exclusive phosphorescence with a PLQY of 3.7% in dichloromethane under ambient conditions. Theoretical simulations reveal a larger singlet (S1)–triplet (T1) gap in Au28ii than that in Au28i, and the higher T2 state plays a critical role in both isomers’ photophysical processes. The insights derived from this investigation not only contribute to a more profound comprehension of the fundamental principles underlying the photoluminescence of atomically precise gold nanoclusters but also provide avenues for tailoring their optical properties for diverse applications.


INTRODUCTION
−15 Various strategies, such as aggregation-induced emission, ligand engineering, intracluster interaction, staple motif tailoring, and heteroatom doping, have been reported with mechanistic insights; 16−23 however, the detailed PL mechanisms still remain elusive in many cases.Thus, exploring NIR PL and understanding the mechanisms are an ongoing effort.
From the structural point of view, deciphering the underlying PL mechanisms for isomeric NCs (e.g., the same core but different surface motifs) can offer deep insights into the structural factors that affect the PL.The importance of structure−property correlations is evident in fundamental studies.While structural isomerism has been well studied in molecules, 24 it remains challenging in nanomaterials 25 due to the lack of atomic precision and detailed structures.−29 The quasi-isomerism phenomenon in thiolate (SR)protected Au 28 (SR) 20 (i.e., different R groups) was observed in 2016, and stable structures were obtained with cyclohexanethiolate (abbrev.CHT) and p-tert-butylbenzenethiolate (abbrev.TBBT) as the protecting ligands. 30Single-crystal Xray diffraction (SCXRD) revealed that both structures of Au 28 (SR) 20 (SR = CHT and TBBT) share the same core (i.e., a four-tetrahedral Au 14 core) but have differences in the surface staple motifs.Specifically, the Au 28 (CHT) 20 is protected by four Au 3 (SR) 4 trimers and two Au(SR) 2 monomers, whereas Au 28 (TBBT) 20 is protected by two Au 3 (SR) 4 trimers and four Au 2 (SR) 3 dimers. 30The study on the PL−structure correlation in Au 28 (SR) 20 (CHT vs TBBT) was carried out in 2020 by Chen et al., who found that Au 28 (CHT) 20 was ∼15-fold higher in photoluminescence quantum yields (PLQY) than Au 28 (TBBT) 20 . 31 2020, Wu's group observed an intriguing structural oscillation in Au 28 (CHT) 20 and discovered true isomerism in Au 28 (CHT) 20 , named Au 28i (CHT) 20 and Au 28ii (CHT) 20 . 26hey obtained the structure of Au 28ii (CHT) 20 from SCXRD, which was same as that of Au 28 (CHT) 20 reported by Chen et al., 31 but no good crystal for Au 28i (CHT) 20 was obtained; rather, they obtained good crystals of CPT (CPT = cyclopentanethiolate)-protected Au 28 , i.e., Au 28 (CPT) 20 , which had the same structure as that of Au 28 (TBBT) 20 .They found that the calculated UV−vis−NIR spectrum of Au 28i by assuming the structure of the CPT-protected one was consistent with the Au 28i experimental spectrum; thus, Au 28i was believed to adopt a similar structure as that of Au 28 (CPT) 20 despite the CPT versus CHT ligand difference.Interestingly, in these two isomers, Au 28ii was found to be more luminescent under ambient conditions than both Au 28i and Au 28 (TBBT) 20 , 26 which implied that the surface motifs played a significant role in the PL.From the structural determination, Wu's group reported that the trimeric staple should contribute more to the emission intensity compared with the dimeric staple and the monomeric staple due to the increasing rigidity, which can reduce the low-energy motion and thus increase the PL emission.In a related work, Pei's group recently performed density functional theory (DFT) simulations to unravel the PL origin in the Au 28 (CHT) 20 and Au 28 (TBBT) 20 quasi-isomers. 27espite the observation of different PL values in the two Au 28 (CHT) 20 isomers, there has been no in-depth investigation into the complex PL mechanism.This knowledge gap prompted our interest to delve deeper and establish a comprehensive PL mechanism in this isomeric system, particularly because the roles of the core and staple motifs in isomeric NCs can be separated and studies on the NIR emission in isomeric NCs are still scarce.
In this study, we investigate the PL mechanisms of the isomeric Au 28 (CHT) 20 NCs by performing time-resolved PL, cryogenic spectroscopies, triplet state probing, and theoretical calculations.Both experiment and simulations reveal an effective reverse intersystem crossing (ISC) (T 1 to S 1 ) in the Au 28i isomer and accordingly concurrent TADF and phosphorescence emission, but the Au 28ii isomer emits phosphorescence only due to the suppressed T 1 to S 1 conversion.In temperature-dependent studies, although Au 28ii is more emissive than Au 28i at room temperature, the vibration-induced nonradiative decay in Au 28i is more efficiently suppressed at 80 K, making Au 28i significantly surpass the PL intensity of Au 28ii , the latter only showing a modest enhancement as its low-frequency vibrations are not effectively suppressed at 80 K. DFT simulations show that the low and high frequency vibrations are associated with the long and short staples, respectively.The obtained PL and structure correlations will be helpful for enhancing the PL in other NCs.

RESULTS/DISCUSSION
Previous work by Wu's group has established the structures of the two Au 28 (CHT) 20 isomers by SCXRD. 26Both possess the same Au 14 core (with an average shortest Au−Au bond distance of ∼2.8 Å), but each has a different arrangement of staple motifs.Specifically, the shell of Au 28i comprises two trimers and four dimers (Figure 1a−c), whereas the shell of Au 28ii comprises four trimers and two monomers (Figure 1d− f).We followed the synthesis reported by Wu's group and prepared the isomeric NCs.The chromatography-purified NCs were then used in the PL studies, as discussed below.
Photophysical Studies of the Isomeric Au 28 (CHT) 20 NCs under Ambient Conditions.The optical absorption and photoluminescence spectra of the two isomeric Au 28 (CHT) 20 NCs in diluted dichloromethane (DCM) solutions were examined under ambient conditions, as illustrated in Figure 2. The Au 28i NC (Figure 2a) displays absorption bands at 446 and 545 nm, whereas the Au 28ii NC (Figure 2b) exhibits absorption bands primarily at 468 and 518 nm, and a shoulder at 580 nm is also found.Both NCs show almost the same optical gap (E g ) of 1.8 eV (Figure S1 inset).
The two isomers display near-infrared PL centered at 813 (1.53 eV) for Au 28i and 850 (1.46 eV) for Au 28ii upon excitation at 365 nm (3.40 eV).The agreement between the absorption and PL excitation (PLE) profiles (Figure 2, dashed lines) affirms that the luminophores are the Au 28 (SR) 20 NCs, rather than any impurity, and that the PL originates from the E g gap in the core.The PLQY were determined by a relative method using the Au 25 rod as a standard (its PL at 900 nm and PLQY ∼8% under ambient conditions in solution). 32The difference in PLQY (∼11.6-foldenhancement from Au 28i to Au 28ii ) at room temperature suggests a notable role of staple motifs (Figure 1b vs e) in the PLQY since their cores are the same.The presence of more trimeric staples in Au 28ii , compared to dimeric and monomeric staples, should contribute to the higher PLQY, which is likely due to the increased rigidity. 26he PL lifetimes were determined by time-correlated single photon counting (TCSPC), which also exhibits a distinct difference between Au 28i and Au 28ii (Figure 3a) at room temperature under ambient conditions.Au 28i shows two lifetime components: 168 ns (10.6%, τ 1 ) and 1379 ns (89.4%, τ 2 ) (average τ av : 1251 ns), note that the percentage in the parentheses indicates the relative amplitude of the component, whereas Au 28ii showed only one component (τ: 2281 ns) (Table 1); note that the lifetime of Au 28ii is consistent with the 1.7 μs from previous nanosecond transient absorption measurements, 31 and the ultrafast 11 ps component in previous femtosecond transient absorption measurements pertains to structural relaxation. 31The results signify different relaxation pathways in Au 28i and Au 28ii .The radiative rate constant (k r ) and nonradiative rate constant (k nr ) for the two NCs were calculated.For Au 28i , k r ∼ 0.3 × 10 4 s −1 and k nr ∼ 8.0 × 10 5 s −1 , and for Au 28ii k r ∼ 1.6 × 10 4 s −1 and k nr ∼ 4.2 × 10 5 s −1 (Table 1).The significantly higher (5-fold) k r in Au 28ii along with a lower nonradiative relaxation rate by a factor of two both makes Au 28ii more luminescent.This also correlates    with the greater flexibility in Au 28i staple motifs giving rise to the higher nonradiative rate due to the vibrational energy loss through the staple motifs.The comparison of their k r and k nr suggests that the QY enhancement in Au 28ii with more trimeric staple motifs is mainly associated with the suppression of nonradiative relaxation and the enhancement of radiative relaxation (Figure 3d).We further tested the PL sensitivity of the two isomeric Au 28 (SR) 20 NCs (dissolved in deuterochloroform) to O 2 .Deuterochloroform exhibits weaker NIR absorption (i.e., vibrational overtones) compared to other solvents, which can reduce solvent absorption-induced distortion of the NIR PL spectra. 12The overall integrated intensity of PL was reduced in both NCs under pure O 2 compared to He, and the appearance of singlet oxygen ( 1 O 2 ) PL signal at 1272 nm (a sharp phosphorescence peak) can be readily observed in both NCs (Figure 3b,e insets), implying the existence of triplet state population in both NCs and their sensitization of triplet oxygen (the ground state of O 2 ) to singlet oxygen (the excited state).The triplet state population of the isomeric Au 28 (SR) 20  NCs implies that their emission can be phosphorescence and/ or TADF 16,33−36 because both types of PL are originated from the population in the triplet excited state (T 1 ).In the deuterochloroform solution of Au 28i , its average PL lifetime (τ av ) is 1320 ns [components τ 1 = 95 ns (8.1%) and τ 2 = 1428 ns (91.9%)] under He and is decreased to 960 ns [component τ 1 = 80 ns (11.7%) and τ 2 = 1077 ns (88.3%)] under O 2 atmosphere (Figure 3c).In the case of Au 28ii (CHT) 20 , its sole PL lifetime τ = 2447 ns (100%) under He is decreased to 1588 ns (100%) under the O 2 atmosphere (Figure 3f).
In regard to the two lifetime components in Au 28i , τ 1 can be tentatively assigned as fluorescence as it did not change much from the He to O 2 atmosphere, whereas the lifetime τ 2 should be the triplet-state emission due to its microsecond timescale and sensitivity to O 2 .For the case of Au 28ii , its only long lifetime component (τ) should be solely triplet state emission because τ in Au 28ii decreases from 2.4 to ∼1.6 μs from He to O 2 , which validates its phosphorescence nature.
Temperature-Dependent PL of the Isomeric Au 28 (SR) 20 NCs.To gain further insights into the origin of PL, temperature-dependent PL spectra for the isomeric NCs were measured from room temperature (rt, 290 K) down to 80 K.The NCs were dissolved in 2-methyltetrahydrofuran (2-MeTHF) to form clear "glass" at cryogenic temperatures for optical measurements.The NCs showed no noticeable degradation after cryogenic measurements, as evidenced by superimposable UV−vis spectra before/after the tests (Figure S2).
With a decrease in temperature, the PL peak of Au 28i significantly intensifies and also becomes sharper (Figure 4a).The PL peak position shows initially a general redshift from rt to 120 K, but then a blueshift as the temperature decreases further to 80 K (Figure 4b).This trend indicates the presence of TADF in Au 28i . 1 The integrated intensity of the PL peak increased monotonically by 2.4 times from rt to 200 K, and further increased by almost 88.2 times from 200 to 80 K; thus, the overall PLQY almost increased by 212 times from 290 to 80 K, that is, from 0.3 to 63.5%.The PL lifetime becomes much longer at low temperatures (Figure 4c), indicating a significant suppression of the nonradiative relaxation by staple vibrations.Overall, the drastic enhancement of the PLQY at 80 K is contributed by the significant suppression of the staple vibrations along with an increase of the radiative rate at low temperature (Figure 4d).
In the case of Au 28ii (Figure S3a,b), as the temperature decreased, the peak position remained unchanged from rt to 220 K and then red-shifted from 220 to 120 K, and finally a slight blueshift down to 80 K.The PL lifetime became longer with decreasing temperature (Figure S3c).In this case, as only one emission lifetime is observed throughout the temperature range from 290 to 80 K and the lifetime is of microseconds, we assign this decay pathway as phosphorescence.Unlike Au 28i , there is no TADF in Au 28ii due to the lack of an additional short lifetime component and any variation with temperature in the PLQY as would be expected for TADF.Upon calculating k r and k nr at different temperatures (Figure S3d), it was observed that in this instance, there is a suppression of the nonradiative pathway without a substantial increase in the radiative pathway.A monotonic increment in PL peak intensity was observed, but only a moderate 3.9-fold enhancement of PLQY was found (Figure S3e).Overall, Au 28ii only exhibits a relatively modest enhancement in PLQY (i.e., from 3.7 to 14.5%) compared to the Au 28i case (Figure 5).
To obtain further insights into whether or not the 212 times enhancement of the Au 28i QY is due to the absorption increase at 80 K, we conducted cryogenic absorption measurements (Figure S4a), which showed only a 17.8% increase in absorbance at 365 nm (the excitation wavelength for PL) from 290 K down to 80 K. Thus, the 212-fold increase is not due to the absorption enhancement but due to the suppression of nonradiative decay by staple vibrations, especially the high frequency vibrations of the four dimeric staples in Au 28i .The temperature-dependent PL excitation spectra (Figure S4b) of Au 28i are essentially unchanged with decreasing temperature, and all show similar spectral profiles, also being similar as that of the absorption spectrum, indicating that the observed PL emission comes from the first excited state (singlet and triplet) over the entire temperature range.
Insights into the PL Mechanism of the Isomeric Au 28 (SR) 20 NCs.The above results show that Au 28i is less emissive at rt (PLQY 0.3%) but becomes highly emissive (PLQY 63.5%) at 80 K (enhanced by 212 times) and, hence, very sensitive to temperature.In contrast, Au 28ii is more emissive at rt (PLQY = 3.7%) but only exhibits a modest enhancement (3.9 times, PLQY = 14.5%) at 80 K, thus being not as sensitive to temperature.To gain further insights into their differences, we further carried out cryogenic absorption measurements on Au 28ii for comparison with Au 28i .The cryomeasurements can probe the vibrational modes that are coupled to the electronic transitions.
In the temperature-dependent absorption spectra of Au 28i (Figure S4a), an obvious blueshift (in wavelength) of the HOMO−LUMO transition peak (i.e., E g ) is evident for Au 28i as the temperature decreases from rt down to 80 K.In contrast, the corresponding gap value for Au 28ii remains nearly constant (Figure S5a).Generally speaking, the renormalization of the E g value is a direct reflection of the electron-vibration interaction in the materials.The large blueshift of the gap in Au 28i suggests a high-frequency vibrational mode in coupling with the band gap electronic transition, which can be efficiently suppressed as the temperature drops to 80 K. On the other hand, the relatively constant gap value of Au 28ii implies that the coupled vibration is of low frequency and cannot be efficiently suppressed at 80 K, unless lower temperatures are applied.These insights explain the observed temperature-dependent PL in which the PLQY of Au 28i increases rapidly with the temperature drop and reaches 63.5% at 80 K (i.e., the staple vibration-induced nonradiative decay is largely suppressed), while the PLQY of Au 28ii increases much more slowly with the temperature drop because low-frequency vibrations are not effectively suppressed at 80 K unless lower temperatures are applied.
Based on the above discussions, for Au 28i , the ∼256 ns lifetime at rt under a N 2 atmosphere (Table S1) should originate from the radiative relaxation of the first singlet excited state (S 1 ), while the ∼1.4 μs should be from the radiative relaxation of the first triplet excited state (T 1 ), and for Au 28ii the sole lifetime component (∼1.9 μs) should be from the radiative relaxation of T 1 .Based on the temperature-dependent PL decay curves in the 290 to 80 K range (Figure 3c for Au 28i and Figure S3c for Au 28ii ), the fitting results for both NCs are listed in Table S1.In the case of Au 28i , as the temperature decreases from rt down to 120 K, the two components become longer and the percentage of τ 2 increases rapidly, while the percentage of τ 1 decreases.When the temperature is lower than 120 K, only one component (i.e., τ 2 ) remains, whereas the other radiative process (i.e., τ 1 ) is suppressed completely; thus, we ascribe the observed τ 1 to a TADF process.On the other hand, for Au 28ii , only one component (τ) was found throughout the temperature range from 290 to 80 K and it became longer at lower temperatures.
Thermally activated delayed fluorescence (TADF) necessitates effective ISC (S 1 to T 1 ) and an extremely narrow gap (<0.2 eV) between S 1 and T 1 .This enables thermal energy to replenish the S 1 state through an "uphill" transfer of the T 1 population, a process known as reverse ISC (RISC). 37,38The occurrence of TADF in Au 28i suggests the proximity of the S 1 and T 1 states and efficient population of these states above 120 K.However, below 120 K, the RISC process was suppressed due to insufficient thermal energy, and the emission mainly arises from the triplet state (i.e., phosphorescence) in the Au 28i .Here, a pertinent question arises: why was TADF not detected in Au 28ii ?To address this and also gain a deeper understanding of the PL mechanism, DFT and time-dependent DFT (TD-DFT) calculations were conducted on these isomeric Au 28 (CHT) 20 NCs.
Theoretical Calculations and PL Mechanism of the Isomeric Au 28 (SR) 20 NCs.−51 Here, the structures of the ground state (S 0 ) and excited states (S 1 , T 1 , and T 2 ) of the two isomers of Au 28 were optimized by DFT and TD-DFT methods.The energy difference between S 1 and T 1 , ΔE S−T (S 1 −T 1 ), were obtained.It is found that the calculated ΔE S−T (S 1 −T 1 ) value of Au 28i (0.126 eV) is smaller than that of Au 28ii (0.240 eV); thus, we expect efficient RISC in Au 28i but not in Au 28ii .For both Au 28i and Au 28ii , the main contributions to S 1 → S 0 and T 1 → S 0 transitions originate from the LUMO → HOMO transition (Table 2).Therefore, we computed the centroid distance between the HOMO and LUMO orbitals (Figure S6).The Au 28i shows a larger HOMO−LUMO centroid distance (0.53 Å) than Au 28ii (0.15 Å).The latter smaller HOMO−LUMO centroid distance results in a greater electron exchange interaction in Au 28ii , leading to a larger ΔE S-T value, while the larger HOMO−LUMO centroid distance yields a smaller ΔE S-T value for Au 28i .
In Table 3, the computed spin orbit coupling matrix element (SOCME), the state energy difference ΔE, ISC, RISC, IC, and RIC rate constants of Au 28i and Au 28ii are presented.Due to the significant relativistic effect of Au, the SOCME between S 1 and T 2 , T 1 are large for both Au 28i and Au 28ii .Such large SOCME values result in very fast ISC processes for Au 28i and Au 28ii .It is worth noting that S 1 → S 0 and T 1 → S 0 possess the same orbital contributions (L → H), which results in a much smaller SOCME between S 1 and T 1 than that between S 1 and T 2 ; this can be understood from the El-Sayed rule.
For Au 28i , the computed S 1 → T 1 and S 1 → T 2 ISC rate constants k S T   attributed to larger SOCME of S 1 and T 2 (221.99 cm −1 ), which indicates that the S 1 → T 2 ISC process is very favorable.After the ISC from S 1 and T 2 , an internal conversion (IC) process is expected from T 2 → T 1 .Here, the computed T 2 → T 1 IC rate constant k T T 2 1 is 1.75 × 10 11 s −1 .Starting from T 1 , the direct emission from T 1 → S 0 , the RISC from T 1 → S 1 , and the reverse internal conversion (RIC) processes are further studied.In Table 3, we display the computed rate constants of these processes.Due to the relatively large energy difference between T 2 and T 1 (0.261 eV), the T 1 → T 2 RIC rate constant k T T 1 2 (1.45 × 10 7 s −1 ) is much lower than k T T 2 1 (1.75 × 10 11 s −1 ).The computed k T S 1 1 of Au 28i (2.9 × 10 10 s −1 ) is very close to the k S T 1 1 (3.53 × 10 10 s −1 ) and far greater than the phosphorescence radiative rate constant k P (T 1 → S 0 , 4.61 × 10 3 s −1 ).Taking these computed rate constants together, an indirect conversion path from S 1 to T 1 by way of T 2 , namely, S 1 → T 2 → T 1 process is suggested (Figure 6a).−54 For Au 28ii , because of relatively small SOCME (1.05 cm −1 ) and relatively large ΔE S−T (S 1 −T 1 ), the S 1 → T 1 ISC rate constant k S T 1 1 is only 2.14 × 10 4 s −1 , which is lower than the fluorescence radiative rate constant k F (5.71 × 10 5 s −1 ), indicating very low possibility of S 1 reaching T 1 through the ISC process.However, a very small ΔE S−T between S 1 and T 2 (0.051 eV) induces a large S 1 → T 2 ISC rate constant k S T 1 2 (1.92 × 10 13 s −1 ), which is beneficial to the S 1 → T 2 ISC process (Figure 6b).Subsequently, similar to Au 28i , the larger ).This situation will make the T 1 → S 1 RISC process unable to compete with the S 1 → T 1 ISC and the T 1 → S 0 phosphorescence radiative process.A direct phosphorescence radiation process eventually resulted in the Au 28ii .
Taking the above discussions together, it is found that the T 2 state plays a significant role in the photophysical processes of Au 28i and Au 28ii .The fast S 1 → T 2 ISC provides an indirect pathway from S 1 to T 1 , including the ISC from S 1 to T 2 first and then from T 2 to T 1 via an IC process.However, due to the different ΔE S−T (S 1 −T 1 ) values in Au 28i and Au 28ii , the RISC rate constants of T 1 to S 1 of the two NCs differ greatly, which leads to different luminescence mechanisms (Figure 6).Based on the computed rate constants, we plotted the dynamic evolution diagram for Au 28i and Au 28ii (Figure 7).The dynamic evolution diagram can better describe the photophysical processes of Au 28i and Au 28ii and explain the differences in their photoluminescence types.For Au 28i and Au 28ii , since the , T 2 population accumulates rapidly with S 1 population decreasing rapidly; then, the T 2 population decreases gradually with the increase in T 1 population.Finally, the T 1 population gradually decreases to 0, with the S 0 population increasing to 1.For Au 28i , the increase in the S 0 population is primarily attributed to the S 1 → S 0 radiative transition, indicating a fluorescence process, specifically TADF emission (T 1 → S 1 → S 0 ).Additionally, the ratio of the S 0 population arising from S 1 radiative transition compared to that from T 1 transition is 2500:1, suggesting the coexistence of T 1 → S 0 phosphorescence.For Au 28ii , the increase in the S 0 population is primarily due to T 1 → S 0 radiative transition, with the ratio of the S 0 population from S 1 radiative transition to that from T 1 transition being 6:1,000,000.To understand the significant difference in phosphorescence efficiency between Au 28i and Au 28ii at low temperatures, we calculated the Huang−Rhys factor (i.e., electron-vibration coupling strength), the reorganization energy, and the Dushinsky matrix for the T 1 → S 0 ISC process.Unlike Au 28i , the case of Au 28ii exhibits significant reorganization energy contributions for S−Au and S−C stretching vibrations (Figure S7).As the temperature decreases, high-frequency vibration modes are suppressed, while in the low-frequency region the Dushinsky rotational effect of Au 28ii is larger than that of Au 28i (Figure S8).Consequently, at low temperatures, the nonradiative processes of Au 28ii cannot be effectively suppressed, resulting in a significant difference in phosphorescence efficiency between Au 28i and Au 28ii .

CONCLUSIONS
In summary, this work reports the structural isomeric effects on the near-infrared PLQY in the two isomeric Au 28 (CHT) 20 nanoclusters, and their PL mechanisms are established through a combined analysis by experiment and DFT/TD-DFT simulations.The Au 28i exhibits a low PLQY (0.3%), whereas its structural isomer Au 28ii exhibits a relatively high PLQY (3.7%) at room temperature under ambient conditions in solution, but at cryogenic temperatures, the vibration-induced nonradiative relaxation is more effectively suppressed in Au 28i than in Au 28ii , leading to a switch of the order of PL intensity, that is, a ∼212-fold enhancement for Au 28i (PLQY: 63.5%) at 80 K versus merely a 3.9-fold enhancement in Au 28ii (PLQY: 14.5%) at 80 K. Temperature-dependent PL measurements along with theoretical calculations reveal both TADF and phosphorescence emission in Au 28i but sole phosphorescence in Au 28ii due to their different S 1 −T 1 gap energies.In both NCs, theoretical simulations indicate a very efficient indirect S 1 → T 2 → T 1 conversion process, but efficient RISC occurs only in Au 28i due to its favorable S 1 −T 1 gap (<0.2 eV), hence, concurrent TADF and phosphorescence in Au 28i , in contrast with the sole phosphorescence in Au 28ii due to suppressed RISC because of a larger S 1 −T 1 gap.Overall, this work presents a paradigm for investigating the complex PL mechanism via a combined experiment-theory approach, and the obtained mechanisms and isomeric effect in enhancing NIR-luminescence will promote the design of NIR emitters and the development of their applications in sensing, bioimaging, photonics, and other fields.

METHODS/EXPERIMENTAL SECTION
The synthesis and isolation of isomeric Au 28i and Au 28ii nanoclusters followed a literature protocol. 26Spectroscopic characterization includes optical absorption, steady-state, and time-resolved photoluminescence, as well as cryogenic spectroscopy.DFT simulations were carried out.Details are provided in the Supporting Information.

* sı Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnano.4c06702.Details of the synthesis, spectroscopic measurements, and theoretical simulations (PDF)

Figure 2 .
Figure 2. (a,b) Optical absorption spectra (black lines; absorbance shown on the left y-axes), PL spectra (solid colored lines; intensities on the right y-axis), and excitation spectra (dotted lines; intensities on the right y-axis) of Au 28i (CHT) 20 and Au 28ii (CHT) 20 , respectively.Other conditions: NCs in DCM, under ambient conditions; slit width for PL measurements: 5/5 nm, and for PLE measurements: 8/8 nm.PL spectra were collected at 0.7 optical density at 365 nm (the excitation wavelength).

53 1. 46 a 1 .c 1 .Figure 3 .
Figure 3. (a) PL decay profiles of the isomeric Au 28 (SR) 20 NCs under ambient conditions in DCM.(b,c) PL spectra and decay profiles of Au 28i in deuterochloroform under a helium atmosphere and O 2 atmosphere, respectively (excitation: 365 nm).(d) Plot of radiative decay rate constant (blue symbol) and nonradiative decay rate constant (red symbol) of the isomeric Au 28 (SR) 20 NCs under ambient conditions in DCM.(e,f) PL spectra and decay profiles of Au 28ii in deuterochloroform under helium and O 2 atmosphere, respectively (excitation: 365 nm).

Figure 4 .
Figure 4. Temperature-dependent data for Au 28i (CHT) 20 .(a) PL spectra in 2-MeTHF of varying temperature from 290 to 80 K. (b) Plot of the PL peak position against temperature.(c) PL decay profiles at selected temperatures.(d) Plot of radiative decay rate constants (depicted in blue) and nonradiative decay rate constants (depicted in red) spanning 80 to 290 K.

Table 1 .
Photophysical Data for the Isomeric Au 28 (SR) 20 NCs in DCM Solution under Ambient Conditions a

Table 2 .
Computed Vertical Emission Energy, Emission Wavelength, and Major Orbital Contributions